Power-limiting method and apparatus



June 1,1926. V 1,586,876

9. E. BUCKLEY POWER max-me ammo AND APPARATUS Filed m 29, '1922 ,y W Hg.2.

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Olive/- 5. Buckley Patented June 1, 1926.

I UNITED OLIVER E. BUGKLEY, OF MAPLEWOUD, NEW JERSEY, ASSIGNOR TOWESTERN ELEC- TRIO COMPANY, INCORPORATED, OF NEW YORK, N. Y., ACORPORATION OF NEW YORK- POWER-LIMITING METHOD AND APPARATUS.

Application filed May 29,

This invention relates to magnetic powerlimiting devices and methods. IThe principal object of the invention is to provide new and improvedmethod and ap- 6 paratus for limiting the power delivered from anelectric circuit to a translating device. Another object of theinvention is to provide for comparatively undistorted transmission to atranslating device of a variable electric current of low power, and toshield the translating devicefrom the effect of a current of higherpower. Another ob ect of i the invention is to guard the user of atelephone receiver against shocks that might be caused by excessivecurrents in the circuit to the receiver, but to facilitate theundistorted transmission of normal voice currents. These and otherobjects of the invention will become apparent on consideration of aspecific example which has been chosen for presentation in'the followingspecification. This embodiment of the invention will now be describedwith the understanding that the definition of the invention is given inthe appended claims. v q 7 L Referring to the drawings, Fig. is adiagram of a telephone receiver and circuit enibodyin the invention, andFig. 2 1s a d agram showing magnetic relations that will be referred toin explaining the principle of the invention, and Fig.3 is an enlargedview of the portion of the curves of Fig. 2 near the origin. I I

' Referring to Fig. 1, the line circuit 11 transmits voice currents fromapparatus 12 to the transformer or repeating coil designated generallyas 13. This transformer 13 has separate windings, a primary 14 and asecondary 15, on the same core 16; The core material is permalloy, acomposition of nickel and iron in certain proportions and heattreated ina certain manner, which will be discussed presently. In circuit with thesecondary winding 15 is the receiver'17.

The nature of the core material'16 and the quantity of it used in thetransformer are such that the repeating coil 13 faithfully reproducesnormal voice currents. But when excessive currents come over the tine11, as for example, if it should become crossed with a power line, thentheower transmitting capacity of the trans ormer 13 is so limited thatno excessive current will be induced in the secondary 15,notwithstanding 1.922.. :Serial-No. 564,428.

the excessive current rush in the primary Accordingly, a listener at thereceiver 17 will have his ear protected against acoustic shock and willnot'be exposed to the danger of injury that might otherwise come to himby the use of a transformer at 13 with hi h capacity for transmittingpower from t e primary to the secondary.

A suitable material for the core 16 is permalloy which may beconveniently used either in the form of wire or thin sheets toaccomplish the results indicated in the foregoing description. Asuitable variety of permalloy is a composition ,of nickel and iron inthe respective proportions of 78%% nickel and 21 iron. Good commercialgrades ofthese ingredients are fused together in an induction furnace,molded into the convenient form of a thick bar or rod and worked down byany of the standard methods of working metals of this general characterto the form of wire or sheets, whichever is desired. The particular'size of wire or thickness of sheets to be used depends, vof course, onthe particular requiredesirable to have the wire sufliciently fine orthe sheets sufliciently thin to minimize losses which would otherwise beintroduced by eddy currents and also to secure the full effect of thehigh permeability of permalloy' by causing the lines of magneticinduction to penetrate through the Whole cross-section of the magneticmaterial, which they would not do if the magnetic material were .in athick form, owin to the well-known phenomenon of shiel ing which occursat high frequencies and which, on account of the high permeability ofpermalloy, is very" marked in this material. A convenient diameter ofthe wire for coils or transformers to be used at voice frequencies isabout 0.010

about 0.003 inch.

The wire or sheet having been worked down to the proper size is next tobe heat treated. If the material is used in the form of wire, theprocedure is to wind. the wire into the form desired for the core of thetransformer. This may conveniently be of the form of a ring.- The wirein this form should be carefully bound together so that in ordinaryhandling subsequent'to the inch. A convenient thickness of sheets isheat treating roeess it will not suffer unnecessary mec amcal straln.vHaving been wound and bound in the proper shape, the core is nextheated to a temperature of about 850 degrces Q. and held at thattemperature for a few minutes to insure that all the wire is at the sametemperature. The core should then be cooled slowly to a tempera ture of600 degrees C. and from that temperature shoul definite rate which willdepend on the size of material to be used and the form in which it isused. This can be easily determined by trials. A convenient means ofsecuring the desired rate of cooling is to quickly withdraw the corefrom the furnace when it has reached the temperature of GOO-degrees 0.,and place it in a blast of air which is controlled to secure the desiredrate of coolin If f. first cutor stamped to the desired form, butprevious to assembling them to form the core of the coil they may beheat treated separately in a manner similar to that described for thewire core, and later bound together. This treatment may be used in thecase of the sheet, since the sheets can be handled and boundtogetherwitl1out.se-

riously straining the material. The permeability of permalloy dependsvery greatly on the condition of mechanical strain. Its initialpermeability at zero magnetizing force may be reduced from values in theneighborhood of 6,000 down to values of theorder of 600 by the mere actof stretching the material. It is, therefore, desirable to carefullyavoid. strains in the finished core.

While 78 7}; and 21 70 have been mentioned as giving the proportion ofthe ingredients of nickel and iron to be employed in making up theimproved magnetic material, it will be understood that the proportionmay deviate considerably from these figures and that it may be desirablefor other reasons to add other ingredients than nickel and iron, such,for instance,'as chromium, which may be used to increase theresistivity, and consequently decrease the eddy current loss in thematerial.

- ity at very The process of making the'transformer which is shownschematically in Fig. 1 having been explained, the principle of actionwhich enables the benefits of the invention to be secured will now beexplained. It is a property ofpermalloy that its permeabillowmagnetizing forces is extraordinarily high, being 0 corresponding valuefor the best grades of iron is only about 300. The value of thepermeability at zero force is obtained by determining a series of valuesfor exceedingly low forces, say of the order of .01

be cooled more rapidly at a' eets are to be used they should be theorder of- 6,000 for zero magnetizing force, whereas the to .05 gauss.The results plot linearly and maycpe extrapolated back to the value forQther important qualities of this composition for t e purpose of 'myinvention are its comparatively low value of magnetization whenmagnetically saturated and the relatively low magnetizing forcerequired" to saturate it. The maximum magnetization of permallo is about10,500 lines per square cm. and it is nearly saturated with amagnetizing force of 1 gauss.

These properties of permalloy are made use of in the following manner.The high permeability at low magnetizing forces is. made useful bydesigning and constructing a transformer with a core of permalloy ofrelatively small cross-section. Since the ini tial permeability is abouttwenty times that of iron, approximately the same efliciency at lowvoltages, so far as limits imposed by magnetic properties are concerned,may be secured with a small permalloy core as with an iron core oftwenty times the crosssection. Now, by virtue of the small crosssectionand 'small value of magnetization when saturated, the latter being onlyonevhalf that for iron, the transformer with the permalloy core is onlyable to transfer about one-fortieth (or less depending upon the mode ofapplication of the electromotive force) the energy from the primary tothe iron core, when both are'operated with high voltages which give highvalues of magnetic field strength. Thus the transformer with the permaloy core is fully as eflicient as that with the iron core for 'lowvoltages but much less efiicient for high voltages, and accordingly1serves as an energy limiting device for igh voltages.

The invention may be better understood by referring to Figs. 2 and 3,which show B-H diagrams P and ll for permalloy and for a good grade ofiron, respectively, the latter material being about the best magneticmaterial available before permalloy was known. The initial permeabilityis given by the slope of the tangent line 0T at the origin, and as shownon the diagram this is very much steeper than the corresponding tangentOT for the iron curve. However, as the diagram shows, the magnetizationof the iron is greater than that of the permalloy for field strengthsgreater than about 1.6 gauss and the ultimate ma netization (not shownon the figure whic only oes to 4.0 gauss) is about twice as great %oriron as for ermalloy.

Let the ratio of t e trigonometric tangents'of the angles XOT and XOT beN.

The curves 'P and I in Figs. 2 and 3 have lit magnetizing netizing forceare applied, then B units of magnetic flux will be developed per unitcross-section. Let us deal with a smaller cross-section of permalloysuch that its area is l unit. This will give the curve P Whose ordinatesare times those of the solid line curve for permalloy, but it will makethe initial ratio of total magnetic flux 4; to magnetizing force H thesame for the permalloy as for the iron. Hence for low magnetizing forces(ampere turns), the small core of permalloy will carry the same flux andgive the same induction ina secvention, is made about times thatcrosssection. This transformer with the small permalloy core will thenbe equivalent to the larger iron core transformer for the lowestmagnetizing forces, and will be more eflicient than the iron coretransformer up to field strengths of about 0.4 to 0.6 gauss where thepermalloy curve P intersects the iron curve I. Beyond this point, forfield strengths greater than 0.4 to 0.6 gauss the iron core transformerwould be much more efficient than the one having the permalloy core.Thus for the normal operating range, which is, by proper design of thetransformer, at less than 0.4 to 0.6 gauss, the permalloy coretransformer is fully as eificient as the iron core transformer, but forhigher forces, where for. purposes of protection w efficiency isdesired, the permalloy core transformer is very much less eflicient thanthe iron core transformer and is accordingly incapable of transferringso much energy to the secondary circuit.

This improved current limiting device has the advantage over a fuse orautomatic circuit breaker in that the latter must first experience acurrent of excessive energy before it will act and limit the furthertransmission of energy, whereas the device of this invention i incapableat any stage of transmitting an excessive energy. The rush of energythat produces operation of a fuse or automatic circuit breaker may domuch damage beforethe device interrupts the current,

The invention claimed is:

1. Means to "protect the user ota telephone receiver against acousticshock, con- I sisting of an 1nterposed transformer in the voicecurrent-circuit, with separate windings and with a small core of higherinitial permeability than iron and lower value of mag netization atsaturation. I

2. In a telephone s stem, means for preventing acoustic shocli neticcurrent limiting device having a core of magnetic alloy consistingchiefly of nickel and iron and having higher initial permeability thaniron and lower magnetization at saturation.

3. ln a si aling system, a receiving in strument an means for preventingabnormally large currents from reaching said instrument comprisingcurrent limiting means having a magnetic core of an alloy consistingchiefly of'nickel and iron, said core having higher initial permeabilitythan iron and lower magnetization at saturation.

4. In a telephone system, means for preventing acoustic shock,comprisinga magnetic current limiting device havin a core of alloyconsisting chiefly of nic el and iron and in which the nickel componentpredominates, said core having higher in tial permeability than iron andlower magnetization at saturation. 3

5. A magnetic power limiting device com prising a coil and a coretherefor of mag netic alloy consisting chiefi of nickel and iron, saidcore having a hig er permeability than ironfor magnetizing forces of a..few tenths of a gauss and having lower mag-=' netization than iron atsaturation.

6. In a telephone s venting acoustic shocr comprising a transformerinserted between the line and the receiving instrument and having a coreof an alloy com rising the magnetic elements. iron and nic el, thenickel component being within a few percent of 78 of the magneticelement content. v

In witness whereof, I hereunto subscribe my name this 26th day of May A.D., 1922.

onrvnn E. BuoKLnY.v

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, comprising a magstem, meansv for pre- 2 I

